In the past it has often been desired to mold plastic parts with a straight, relatively small diameter hole through it. An example is a medical catheter in the form of a jacket or sleeve of plastic material through which a medical needle is to be inserted. If the hole is not of constant diameter or is somewhat misaligned, upon insertion of the needle into coaxial relation within the plastic sleeve or jacket, the tip of the needle may pierce the side wall or shave some of the material from it which results in a combination which is not usable. For example, if some small plastic shavings enter the needle, it cannot be used for injection into the arm of a patient because the shavings would enter the blood. There are numerous other stiuations in which a relatively fine hole is required through a tubular plastic molded part, whether the fine hole is coaxial with the part or not, or whether the part is cylindrical or not; and generally the term tubular as used herein refers to parts which have at least one fine through hole. Oftentimes such molded plastic parts are required to be of a thin wall of constant thickness with a longitudinally extending fine hole through it. In the past, when the plastic is flowed into a cavity about a fine core, that is a core of small diameter, the pressure under which the plastic must be held in order to flow through the runner and into the cavity during the molding operation, tends to cause a thin mold core to wander from perfect or near perfect axial alignment rendering a defective part. For this reason, the prior art has been limited to relatively short tubular lengths in relation to the diameter of the hole through it. If for example the tube is to be relatively long, a small hole, especially a small hole in a thin walled tube is difficult or impossible to make. Different materials provide different problems. For example, polyurethane plastic material is somewhat difficult to mold into a thin walled plastic molded part with a fine diameter hole axially through it. Similarly, problems occur in molding thin walled plastic parts with a fine diameter hole through it which are not of rigid plastic but, rather, a relatively bendable plastic material such as is used in catheters for medical devices. The same is true of polypropylene materials as well as the commercially available plastic material known as Teflon manufactured by and made available commercially by the E. I. duPont de Nemenours Company. For example, it is often required to make small diameter holes of about 0.020 inches in a tube of plastic of about an inch or more in length with an outside diameter of about 0.032 inch so that the wall is relatively thin and in the range of about 0.006 inch in thickness. Oftentimes it is desired that the hole or passageway through the molded plastic part be about 0.002 inch to about 0.100 inch in diameter. The difficulty has been that the core is relatively fine in such molding operations as a necessity and it will not stay centered within the cavity of the mold when the mold closes and is under pressure.
In the past, some plastic parts have been made which have small holes through them. This has been done by drilling with a fine drill or by a laser beam and the length of the tube with the fine hole is of a limited length. Also it is known to extrude a tube and draw it to a smaller diameter. The former presents limitations as to the thickness requirement of the tube wall and the latter is limited to a hole which is cylindrical, that is, it cannot be internally stepped for example. Generally such prior art devices would not be practical in quantities in a production setting.
The present invention provides a process generally for molding a relatively long generally tubular plastic piece; usually with a thin wall, and having a relatively small diameter axial hole through it, which may not be necessarily cylindrical, e.g., it may be stepped; and the process consists of positioning mating mold parts about a thin wire core to define a mold having an annular cavity about the wire and a runner means to flow plastic into the mold cavity and includes the step of maintaining the mold parts in mold defining relation about a relatively thin wire which extends from the mold and through the cavity which wire is subjected to tension while simultaneously flowable plastic is injected into the cavity through the runner and coaxially about the wire while it is under tension.